The present invention relates to a load limiting method and apparatus for a steam turbine plant and more particularly relates to a method and apparatus for operating a turbine plant having a malfunctioning feed water heater.
In large steam turbine plants, it is usual to provide a plurality of feed water heaters to obtain a high thermal efficiency of the plant. In such feed water heaters, feed water supplied to the boiler is preheated through heat exchange with steam at a high temperature that is extracted from a turbine. Steam extracted from the turbine condenses and becomes drain water in the feed water heater during the heat exchange. This drain water is discharged to a next adjacent feed water heater located on the upstream side of the feed water heater with respect to the flow of feed water and recirculates to the boiler through a deaerator and a condenser.
Under a normal operating condition, the drain water condenses in the bottom of the feed water heater and is discharged almost at a constant rate by the pressure of the extracted steam and the drain level in the feed water heater is kept almost constant.
In the case where a feed water tube ruptures within the feed water heater, for example due to vibration or corrosion by steam, feed water will discharge from the ruptured point of the feed water tube so that the drain level in the feed water heater rises rapidly to result in an unsatisfactory heat exchange. As a more serious problem, it is possible that the drain water will flow back to the turbine through the steam extraction tube, which results in a corrosion of the turbine.
In a prior art method, a check valve is provided between the turbine and the feed water heater to prevent the backward flow of the drain water to the turbine. However this is not reliable to prevent the backward flow of the drain water, because the valve seat or the valve disc of the check valve often becomes distorted by heat.
It has recently become a practice to positively close the extraction stop valve in order to perfectly isolate the feed water heater from the turbine, which extraction stop valve may be provided between the check valve and the turbine. This will prevent the above mentioned backflow.
It has been revealed, however, that when the feed water heater is perfectly isolated from the turbine, there is a serious problem with respect to the strength of the diaphragms of the turbine. When a turbine is macroscopically seen, it is an aggregate of orifices which consists of diaphragms and blades. It is so designed that the pressure drop of the fluid inside the turbine is induced only, for example, at a nozzle situated on the diaphragm and that no pressure drop arises at the blades. The strength of the diaphragm is designed so as to withstand the pressure difference between the fluid pressures at adjacent stages at the time when the control valve is fully opened under the usual steam extraction conditions. The fluid pressure differences between the adjacent stages are successively superposed substantially in proportion to the amount of steam flowing to the succeeding stages.
Accordingly, when a part of the steam extraction of the turbine, which is operating under the normal steam extraction condition with the control valve kept fully open, is stopped due to the abnormality of the feed water heater as previously stated, the fluid pressure distribution in the turbine changes, and the diaphragm portion that has its pressure drop increased as a result thereof can undergo an excessively pressure difference greater than the designed value. At this time, the strength of the diaphragm portion becomes unsatisfactory.